//This is an event that fires when the user presses the device back button
document.addEventListener("deviceready", onDeviceReady, false);

function onDeviceReady() {
    document.addEventListener("backbutton", backKeyDown, true);
}

function backKeyDown() {    
    var cFirm = confirm("Are you sure you want to exit the application?");
    if (cFirm == true) {
        //Code to exit the application
        navigator.app.exitApp();
        navigator.app.back()
    }
}